Many liquids containing suspended solids have been treated in clarifying vessels in which the liquid flows in a upward helical path, such as disclosed in Wyness U.S. Pat. Nos. 4,146,471 and 4,765,891. Typically, clarified water from the clarifying vessel is transferred to a filtering facility in a building nearby.
Filters are space intensive and typically require a sheltering building to protect the filter integrity and finished water quality. All regulatory agencies require a minimum of two filter cells so that one cell is in service while the other is being backwashed or maintained. Backwashing of filter media is necessary when there is a loss of head in the filter or excessive turbidity of filter effluent. Filter cells typically require a backwash source, such as a backwash water storage tank, piping, and backwash pumps.
Filter cell distribution and backwash equipment can be mechanically and hydraulically complicated which can present operating and maintenance problems. Further, some filter designs are compromised by limitations on the available backwash head, such as shallow profile filters which may require an auxiliary backwash source.
Some systems are configured in a way that confines valves, piping, and mechanical and electrical components making maintenance time consuming and difficult. Underdrains for the various cells cannot be isolated so that the failure of one filter cell's underdrain requires all filter cells to be shut down for repair.
Further, individual cells cannot be filtered to waste which is an increasingly important operational procedure for surface water treatment plants. With the Safe Drinking Water Act (SWDA) and the Surface Water Treatment Rule (SWTR) regulations on turbidity, giardia cyst removal, virus removal and cryptosporidium removal, regulatory agencies are requiring that after a backwash cycle, water passing through the media and filter underdrain be sent to waste until the filtered water turbidities are below the regulatory limits. This filter-to-waste system must include an air gap to break any siphoning of waste water from the waste pipe back to the filter cells so that there is no possibility for contamination of those filter cells remaining on line.
Thus, there is a need for a filtering system that reduces space requirements, the capital costs of construction, and the operational and maintenance costs of backwashing the filtration system, while providing easy access to working components, filter cell isolation capabilities, underdrain isolation capabilities, ample backwash head and backwash water storage.